When optical systems use high power lamp light sources, there can be a significant radiative proportion of the light emitting in the ultraviolet region. When organic materials are in the light path, there will be a degradation of this material over time. Ultraviolet radiation also causes degradation and discoloration in such items as paints, fabrics and plastics. Specifically, electromagnetic energy in the ultraviolet spectrum (i.e., between .about.100 and .about.400 nanometers), causes paints and dyes to fade, causes rubber to crack, and plastics to crumble with time. Therefore, strong ultraviolet absorption by architectural glazing materials is beneficial.
The sun is not the only light source that emits ultraviolet radiation. Various artificial lighting sources like Hg or Xe ARC and other discharge lamps emit ultraviolet radiation. Ultraviolet absorbing glasses can be used that block the entire range of the ultraviolet emission of these sources. However, as a result of the absorption, with prolonged usage, many glasses tend to solarize or darken with time, especially from the absorption of the shorter wavelength, higher energy portion of the ultraviolet region.
Ultraviolet absorbing glass trademarked Pyrex-UV-Plus.TM. (available from Corning Incorporated), has applications such as fiber optic distributed lighting, Liquid Crystal Projection and other projection technologies. These lighting or projection systems use high intensity discharge light sources which contain radiated output in the ultraviolet spectrum. The Pyrex-UV-Plus.TM. glass has a very sharp band edge near 400 nm (ultraviolet--visible boundary) which is useful in protecting organic components that degrade under ultraviolet fluence, while providing maximum visible radiation transparency throughout the visible part of the spectrum (400-760 mm). However, some customer and internal testing has shown that certain metal halide lamps which have output in the range of 250-280 nm can cause the absorbing Pyrex-UV-Plus.TM. glass to photodarken in the visible spectrum. Using glass pre-filters that have cutoffs .gtoreq.280 nm can prevent the photodarkening of the Pyrex-UV-Plus.TM. glass. However, adding two more glass surfaces would reduce the visible transmission. Most of the Pyrex-UV-Plus.TM. glass applications in fact use anti-reflection coatings applied to the glass surfaces to improve the visible lumen output.
Co-assigned U.S. Pat. No. 5,925,468, by Stewart, titled Solarization Resistant and UV Blocking Glass, filed Apr. 1, 1997, and herein incorporated by reference discloses ultraviolet-absorbing glass combined with a solarization resistant glass article to provide a substantially complete UV spectral blocking filter.
While these glasses provide a substantial improvement in ultraviolet absorbing glasses, there continues to be a need for improved systems. Accordingly, it is the object of the present invention to provide an improved UV-blocking glass.